Dead Confederate soldiers in a ditch after the Battle of Antietam near Sharpsburg, Md.

Library of Congress via AP

The front side of Confederate Army Gen. Robert E. Lee's Special Order No. 191 dated Sept. 9, 1862. The handwritten document detailed the Southern commander's audacious plans for an invasion of enemy territory that would propel the Confederates to victory. Carelessly left behind as Lee's army marched north, the copy was spotted in a field by Union infantrymen and relayed up the North's chain of command.

Library of Congress via AP

The back side of Confederate Army Gen. Robert E. Lee's Special Order No. 191 dated Sept. 9, 1862.

Library of Congress via AP

President Abraham Lincoln and Gen. George B. McClellan sit in the general's tent after the Battle of Antietam near Sharpsburg, Md, in 1862. McClellan's skill in organizing and preparing troops was what made Lincoln elevate him to command, even though the president had long been frustrated by another defining trait of "Little Mac" - his paralyzing deliberateness and tendency to grossly exaggerate the forces he faced. As a general, he was the temperamental opposite of Gen. Robert E. Lee.